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b=bDYiEGuttiQVberspseQVpB4Jch8Z2ZrWiIOeyuRcCuXGLIRFXR275xuUlcX94Rzs 2/dyIVxOvyxpZIPwZdNu23hhgP9dZDWhSjr9qzCf2iQoOehtppGlBXg73XU43rKEh0 yoiwQLvMhLXPwFvfwW93oJvFme797H7Hkmvd5C30LUVn74wIU4Nhr0vtFk2QK8+fxj XEUmBXgmsdvLP6hABt/noHrUNTGW3thG6vySSeZFw+ZATCYgTBA50t8VTxWRnmMzuY cu1XWeh8zFpfjxAsFiZUdQUuVJFRE/G2yRBQ38s8jZ2hSZBkW7+/oJUSKq1E1aLIwu VLyL1riVTmHNw== Date: Tue, 20 Feb 2024 10:56:53 +0100 From: Christian Brauner To: Chuck Lever Cc: viro@zeniv.linux.org.uk, jack@suse.cz, hughd@google.com, akpm@linux-foundation.org, Liam.Howlett@oracle.com, oliver.sang@intel.com, feng.tang@intel.com, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, maple-tree@lists.infradead.org, linux-mm@kvack.org, lkp@intel.com Subject: Re: [PATCH v2 1/6] libfs: Re-arrange locking in offset_iterate_dir() Message-ID: <20240220-ortsrand-initialen-43550ee746ed@brauner> References: <170820083431.6328.16233178852085891453.stgit@91.116.238.104.host.secureserver.net> <170820142021.6328.15047865406275957018.stgit@91.116.238.104.host.secureserver.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <170820142021.6328.15047865406275957018.stgit@91.116.238.104.host.secureserver.net> X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 55E61180029 X-Stat-Signature: 9my4jaoip7qrcqdbgmdbcqqynp367ecs X-Rspam-User: X-HE-Tag: 1708423020-250218 X-HE-Meta: U2FsdGVkX19P19DsebhnNetxfKvCOQNjimafF6pkXlXDRVa4PGidFqtL27LaouTOcuc15PZ+IF4I9+ITlJ2fo4DkHiIfSUz01vdfY+c6csMOD30to5UDZI5icxaqWAqhi7GwtJRZtETCbD/9nEGmzmtU/gp80zvqeKJ3iaOlGLxnpeDvz1Bl0CZWIwmD40kM1bQLrn0p9ODH/qCdXzzpX3oDEWDVQk58O8cnwmIN9IgpMW353HF6roKiXM90HvFnhGhHMZ+Q433uD/1F5w1zij51CLswfFHwkHFsOzF5/x3ibyWu5D5JBQtzpdYjRcPI8kA/OwpzBEvdGSj1Blxc6AAEr7VzuFvwzgUceae/jzyiySbIFuWULRPvePfkDw0mIxu3lXMUaBvuqNEWrI8stYUgUgkqbXoXNfmLHGvut0o4+pygazu0CHB/Lay9xHKU0vkUxgh2BFPpH2JLWqbBr9kI8yuR95w1ddjIAO02ZpfCpxp8mcNaGAax6a7I79qeZhJg0X6KOr6pJO9Np4jXxZ4VialdYh5qkRB6NRKwAu//3sPqfZURsVc0eg33tZRw0VBUVyd+fvec6qqw6JVxU7l/tMQKag/91R9I3vz4Rmi5V1S8C2Rk79F2UNK220bX1xq6H7NznzoAhXXyHhNFu6ObVipMmZW5eiDhAh1OABjeF8/00+G3ATpryu4ek4R6X9VV//rAzP9LwNYr3URNOMtkv6OdAkd0mZjncK29M01UyXgipGykB2zATkl2zYyhI7A4Kaz0JuHuS2JGb77gdJvtw6A/4DbTHGotFk85E1bOpvYQo36kW2cV/jrrGhCknyILNk2MSOuZ/7YG4OZGbhoD4D2K3k6FPQfvSVkUAdvOfQgaEjRFey2kwKmwOug9/AhBV4NSpBg1mx67FsorExNUNXwrPdky2/4LXsQQIPPYdclrJaa0v01jdunntqJAlMgoIg40NztvO5H6+kZ 4QDuF2o3 T39DuDw2d6XMwx1VvaZc3Qf90slWVLRno9MiKdZnk4QQRb/fvoxnpB7P/X7C3Kdv8cYuqkw3JT8jpRngBNHwJVt2RZ/qZeNAhnCOkTodfYsam6+yiMlX5QtPLFcYhqhjSQJpNN+dSUrip7m14mvx3+u3nRW9Q68OTE7apxjXvDaeZFjVEPRD4AaBOW6tgv6fN+TV86NWcI//vDTFqLk4chJ0P9G4Y6UR8gJmk6Q/HhHmBTzpuhhCJuDi6GU6/MlrUQ8Ve X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sat, Feb 17, 2024 at 03:23:40PM -0500, Chuck Lever wrote: > From: Chuck Lever > > Liam and Matthew say that once the RCU read lock is released, > xa_state is not safe to re-use for the next xas_find() call. But the > RCU read lock must be released on each loop iteration so that > dput(), which might_sleep(), can be called safely. Fwiw, functions like this: static struct dentry *offset_find_next(struct xa_state *xas) { struct dentry *child, *found = NULL; rcu_read_lock(); child = xas_next_entry(xas, U32_MAX); if (!child) goto out; spin_lock(&child->d_lock); if (simple_positive(child)) found = dget_dlock(child); spin_unlock(&child->d_lock); out: rcu_read_unlock(); return found; } should use the new guard feature going forward imho. IOW, in the future such helpers should be written as: static struct dentry *offset_find_next(struct xa_state *xas) { struct dentry *child, *found = NULL; guard(rcu)(); child = xas_next_entry(xas, U32_MAX); if (!child) return NULL; spin_lock(&child->d_lock); if (simple_positive(child)) found = dget_dlock(child); spin_unlock(&child->d_lock); return found; } which allows you to eliminate the goto and to have the guarantee that the rcu lock is released when you return. This also works for other locks btw.